1. Field of the Invention
The present invention relates generally to messaging devices or pagers, and particularly to a technique for providing receive signal quality estimates suitable for pagers used in satellite-based communications systems.
2. Discussion of the Known Art
Known messaging or "paging" systems use at least one base station to transmit messages or data to selected system pagers. Pagers and other user equipment assigned to a wireless communication system are generally referred to as subscriber units (SU's). Over a forward or "down-link" frequency channel, a base station transmits down-link signals containing information destined to individual SU's whose addresses are also encoded in the down-link signals.
It is contemplated that a wireless communication system capable of global coverage for all subscribers, will soon be available. The system, known as "IRIDIUM", is presently specified to include mobile telephony and one-way pager service world-wide using a number of communication satellites in low-earth orbit (LEO). A satellite-based pager service will overcome certain limitations of terrestrial based systems. At present, people traveling with pagers know that paging systems around the world are usually not compatible. The United States (900 MHZ FLEX), South Korea (322 MHZ FLEX), and France (VHF ERMES), all have different paging frequencies and protocols. While several paging service providers have achieved regional and even some international coverage, such services still exclude major portions of the world. The result is that paging subscribers have few options for roaming worldwide and, sometimes, even on a regional basis. By contrast, a satellite based system may operate according to a uniform protocol worldwide.
A given pager may not always be capable of receiving a message sent from an overhead satellite, however. For example, the pager may be inside a building that attenuates wireless messaging signals below a certain threshold needed for accurate reception by the pager. Pager reception is also affected by the size of a pager's antenna, which for ease of portability may not extend from the body of the pager itself. If a pager subscriber has the capability to determine when his or her pager is unable to receive messaging signals accurately from a system transmitter, whether terrestrial or satellite based, he or she can take appropriate steps, e.g., move to a more favorable location where received signal strength exceeds the required threshold. The degree to which a pager can decode signals at less than a defined optimum signal strength, is referred to as the pager's "link margin". In the mentioned IRIDIUM system, pager link margins of about 30 dB are presently expected.
So-called "receive signal strength" (RSS) circuits have been provided for terrestrial-based paging systems, to alert users that their pagers may be out of range from a base station transmitter, and that message signals addressed to their pagers may have been missed by their pagers. See, for example, U.S. Pat. No. 5,032,835 (July 1991); U.S. Pat. No. 4,851,820 (July 1989); U.S. Pat. No. 5,049,875 (September 1991); and U.S. Pat. No. 5,289,178 (February 1994). All relevant portions of these patents are incorporated by reference herein.
Because the link margins for pagers in a satellite-based system tend to be less than those for pagers in terrestrial based systems, it is preferable to include some form of RSS indicator on the former so that users can be made aware of current message reception conditions. As mentioned, RSS indicators are implemented in terrestrial-based SUs (including cellular telephone hand sets) so that users can reposition themselves or their pagers for better receive conditions. Due to significant differences between terrestrial and satellite-based communication systems, however, it is not possible simply to incorporate the known RSS measurement schemes into satellite system pagers.
Specifically, in urban areas where RSS indicators are most useful, downlink signals from a nearby terrestrial base transmitter are usually plentiful. Thus, a pager can always monitor an active messaging frequency channel and evaluate reception conditions by measuring the strength of signals addressed to other pagers grouped in a common geographic area or cell. The time needed for such a receive signal strength evaluation is typically only a few seconds.
In a satellite based communication system, individual service areas or cells may not be continuously active, however. Thus, message signal traffic available for reception in a given area at any one time is substantially limited. For example, in the IRIDIUM system 10, each system satellite services 48 contiguous cells. Only a small number of these cells may be active at any given time, and, at certain times, the 48 cells are illuminated by a satellite antenna only sequentially yielding a duty cycle of 1/48 per cell. Accordingly, the number of signals available to make a receive signal strength or quality estimate in any one cell is limited. Also, in the case of satellite based transmissions, a relatively large amount of signal processing is needed to receive and to decode message signals due to, e.g., Doppler effects in the case of LEO satellites. Thus, for satellite system pagers, relatively few opportunities may exist to provide an indication of receive signal quality promptly.
In addition, terrestrial-based cellular communication systems have link margins typically as much as 80 dB, i.e., enough for an RSS indicator to construct a power meter scale. For satellite signals, the limited available transmit power and extremely large path loss typically result in a much lower dynamic range for reception by system SUs. This significantly limits the range of RSS measurements that can be used to construct a power meter scale on satellite system pagers.